Infusion fluid warmer

ABSTRACT

The present invention relates in one aspect to an infusion fluid warmer which comprises a casing shell having an upper wall structure and a lower, opposing, wall structure. The casing shell encloses a fluid channel or passage extending through the casing shell in-between the upper and lower wall structures and fluid inlet and outlet ports coupled to opposite ends of the fluid channel or passage to allow a flow of infusion fluid through the casing shell. A housing shell is formed in a thermally conducting and electrically insulating material and a heating element is bonded to the housing shell and thermally coupled thereto. The fluid channel or passage extends through the housing shell or extends around the housing shell such that heat energy is transferred to the infusion fluid by direct physical contact with housing shell material.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is the National State Entry ofPCT/EP2012/069887, filed on 11 Apr. 2014, which claims the benefit ofU.S. Provisional Application No. 64/546,779, filed on 13 Oct. 2011.

The present invention relates in one aspect to an infusion fluid warmerwhich comprises a casing shell having an upper wall structure and alower, opposing, wall structure. The casing shell encloses a fluidchannel or passage extending through the casing shell in-between theupper and lower wall structures and fluid inlet and outlet ports coupledto opposite ends of the fluid channel or passage to allow a flow ofinfusion fluid through the casing shell. A housing shell is formed in athermally conducting and electrically insulating material and a heatingelement is bonded to the housing shell and thermally coupled thereto.The fluid channel or passage extends through the housing shell orextends around the housing shell such that heat energy is transferred tothe infusion fluid by direct physical contact with housing shellmaterial.

BACKGROUND OF THE INVENTION

Intravenous or infusion fluid such as blood is commonly used inhospitals and in the field for example in emergency or war zones. Theinfusion fluid is important for virtually all medical procedures andapplications. Such infusion is typically delivered from an IV fluid bagor container into a blood vessel of a patient. It is desirable to warmthe blood or IV fluid to a certain range of temperature (e.g. between 36and 37 degrees Celsius) to avoid temperature drop in the patient whichmay lead to hypothermia.

There exist various conventional devices and techniques for heating orwarming infusion fluids before being administrated to a patient.However, these conventional devices and techniques suffer from a numberof drawbacks. The conventional infusion fluid warmers are bulky andheavy which make them unsuited for portable applications where they haveto be transported by foot for example by soldiers, rescue workers orambulance crews to reach inaccessible emergency sites. The heavy andbulky nature of existing infusion fluid warmers also makes it difficultor impossible to secure or attach these to the patient's body in aconvenient and safe manner. Another disadvantage of existing fluidwarmers is that they are composed of a large number of separate partswhich make them expensive to manufacture and tend to reduce reliabilitydue to a multitude of separate engaging parts.

Yet another disadvantage of existing fluid warmers is a lack of amechanism for capturing and coupling heat energy dissipated in theenergy source during fluid warming to the infusion fluid. This leads toinefficient use of energy stored in the energy source, such asrechargeable or non-rechargeable batteries, and therefore a need forlarger, heavier and more costly energy sources than strictly required towarm a given volume or amount of infusion fluid. According to one aspectof the present invention, this problem is solved by conducting excessheat energy generated by a portable energy source to the infusion fluidto heat the latter. Thus, ensuring that the energy held in the portableenergy source is put to efficient use.

WO 2003/049790 A1 describes a system for heating transfusion fluidscomprising a fluid warmer having an inlet channel and an outlet channel.A fluid passage of meandering shape is formed in a separate cartridgearranged between a pair of heat contact plates.

SUMMARY OF THE INVENTION

A first aspect of the invention relates to an infusion fluid warmerwhich comprises a casing shell having an upper wall structure and alower, opposing, wall structure. The casing shell enclosing:

-   -   a fluid channel or passage extending through the casing shell        in-between the upper and lower wall structures,    -   fluid inlet and outlet ports coupled to opposite ends of the        fluid channel or passage to allow a flow of infusion fluid        through the casing shell. A housing shell is formed in a        thermally conducting and electrically insulating material and a        heating element is bonded to the housing shell and thermally        coupled thereto. The fluid channel or passage extends through        the housing shell or extends around the housing shell such that        heat energy is transferred to the infusion fluid by direct        physical contact with housing shell material. The casing shell        of the infusion fluid warmer may comprise a thermoplastic        material or elastomeric compound fabricated by a suitable        manufacturing process such as injection moulding. The casing        shell may function to protect the housing shell from mechanical        shock, impacts and pollutants in the external environment. The        casing shell may have a shape with rectangular, elliptical or        circular cross-sectional profile, i.e. along a cross-section        transversal to the direction of infusion fluid flow at the fluid        inlet and outlet ports.

The fluid inlet and outlet ports of the casing shell allow the infusionfluid to flow through the infusion fluid warmer with cold or unheatedinfusion fluid entering at the inlet port and heated or warmed infusionfluid exiting through the outlet port on its way to the patient.

The housing shell possesses an advantageous multi-purpose role in thepresent infusion fluid warmer by acting as a physical carrier for theheating element bonded thereto, e.g. thick film and/or thin filmresistor(s), and as a heat exchanger or heat plate transmitting heatenergy directly to the infusion fluid by the physical contact betweenthe housing shell material and the fluid. This property is capable ofproviding a compact infusion fluid warmer requiring only a minimum ofseparate parts to be manufactured and assembled. The housing materialpreferably comprises ceramic material such as Aluminium Oxide (Al₂O₃),Aluminum Nitrate or Beryllium Oxide which all are well-suited assubstrate materials for a large range of heating element materials, inparticular thick film and thin file resistors. The ceramic materialadditionally possesses good thermal conductivity and good electricallyinsulating properties. The skilled person will appreciate that thematerial of the housing shell is preferably bio-compatible for example abio-compatible ceramic material such as Aluminium oxide (Al₂O₃) due toits direct contact with the infusion fluid to be distributed to thepatient. Alternatively, materials that are not bio-compatible may beused such as ceramics Aluminum Nitrate or Beryllium Oxide.

To possess adequate electrically insulating properties, the housingshell preferably comprises a material with specific electricalresistance larger than 1×10⁹ ohm*m, to meet official requirements. Topossess adequate thermally conducting properties, the housing shell ispreferably made of a material with specific thermal conductivity largerthan 0.5 W/(m·K), more preferably larger than 1.0 W/(m·K) even morepreferably larger than 10.0 W/(m·K).

The housing shell may be fabricated as a single unitary element forexample by moulding or machining a solid object. The fluid channel mayhave a largely straight rectangular form extending through a centralportion of the element. In alternative embodiments, the housing shellcomprises a plurality of separate structures that are bonded togetherafter individual fabrication for example by gluing, soldering,press-fitting, welding etc. In one such embodiment, the housing shellcomprises an upper wall structure and a lower wall structure formed inseparate upper and lower housing shells, the upper and lower housingshells being bonded to each other. In this embodiment, the fluid channelmay extend in-between the upper and lower housing shells for exampleformed by mating grooves or trenches formed in facing surfaces of theupper and lower wall structures of the housing shell to form a fluidpassage extending through the housing shell. The fluid channel may havea variety of different shapes but preferably a shape that maximizes acontact area between the infusion fluid and the housing shell(functioning as a heat exchanger) to increase the fluid heating capacityof the fluid warmer (for example expressed in liters per minute). In oneembodiment, the fluid channel has a meandering shape in across-sectional plane extending perpendicularly to the flow of infusionfluid at the inlet and outlet ports. This plane may be orthogonally to alongitudinal axis of the housing shell if the latter has a flat platelike structure. In an alternative embodiment, the fluid channelcomprises a substantially straight channel extending along thelongitudinal axis of the plate shaped housing shell. In the latterembodiment, the fluid channel preferably extends through a substantialportion of a width of the housing shell to maximise the contact areabetween the infusion fluid and the housing shell. The fluid channel maybe formed as a substantially rectangular straight tunnel with a heightbetween 0.1 mm and 5 cm such as between 0.5 mm and 2 cm. If the housingshell has the flat plate shaped structure, its height may be less than4.0 cm, preferably less than 1.0 cm.

In yet another embodiment of the infusion fluid warmer, the fluidchannel extends around the housing shell such that the fluid channelcomprises a first channel segment arranged between the upper wallstructure of the housing shell and the upper wall structure of thecasing. The fluid channel further comprises a second channel segmentarranged between the lower wall structure of the housing shell and thelower wall structure of the casing. In this embodiment, the upper andthe lower wall structure of the housing shell are both in physicalcontact with the infusion fluid to provide a large contact area betweenthe housing shell and infusion fluid so as to ensure efficient transferof heat energy. The skilled person will appreciate that the fluidchannel in the alternative may comprise only a single channel segmentextending around the housing shell.

The heating element is preferably bonded to a surface of the upperand/or a surface of the lower wall structure facing away from the fluidchannel to isolate electrical drive voltages or currents supplied to theheating element from the infusion fluid and prevent corrosion attacks onelectrical terminals or components of the heating element. In one suchembodiment, the heating element comprises a thin film resistor or athick film resistor bonded directly on the surface(s) of the upperand/or lower wall structures facing away from the fluid channel forexample by screen-printing or other suitable bonding mechanism. Hence,in these embodiments, the electrically insulating property of the upperand lower wall structure(s) is used to electrically insulate theinfusion fluid from the DC or AC voltage/current applied to heatingelement or elements to heat these. The thick film resistor or thin filmresistor may naturally comprise a plurality of resistor elements orindividual resistors coupled in series or parallel to provide anydesired resistance value depending on the requirement of theapplication. The thick film or thin resistor may cover a considerableportion of the total area of the surface of the upper wall structurefacing away from the fluid channel and/or a considerable portion of thetotal area of the outer surface of the lower wall structure facing awayfrom the fluid channel. The total resistance of the thick film or thinfilm resistor may vary widely e.g. from 0.001 ohm to 10 Kohm. Thethermally conducting property of the upper wall structure and the lowerwall structure ensure that heat energy dissipated in the thick filmresistor is efficiently conducted to the fluid channel. The surface ofthe upper and lower wall structures facing away from the fluid channelmay comprise a pair of electrical coupling terminals for receipt ofelectrical power to the thick film or thin film resistor.

According to a preferred embodiment of the invention, the heatingelement comprises a portable energy source such as rechargeable,non-rechargeable batteries, a super capacitor etc. enclosed between theupper and lower wall structures of the housing shell. This embodimentprovides a fully portable infusion fluid warmer which can be used in thefield for example in emergency or war zones allowing infusion of fluidssuch as blood without cooling the patient. The simplicity of the presentinfusion fluid warmer combined with the small size and low weight is asignificant advantage for medical personnel transporting the warmer.

According to this embodiment, the portable energy source is thermallycoupled to the fluid channel preferably by direct physical contact withthe housing shell to thermally conduct heat energy dissipated in theportable energy source in connection with its depletion to the fluidchannel and infusion fluid flowing there through. In this context“direct physical contact” means in contact without any interveningpassage or layer of atmospheric air or other gaseous substances. Thematerials, shape and dimensions of the housing shell, the fluid channeland the portable energy source are preferably configured such that thethermal resistance between a housing of the portable energy source andthe infusion fluid is less than 100° C./W, preferably less than 25°C./W, even more preferably less than 10° C./W.

The skilled person will appreciate that the heat energy dissipated inthe portable energy source for example due to its internal impedance mayexclusively be utilized to heat the infusion fluid or it may supplementheat energy dissipated in the heating element such as theabove-discussed thick film or thin film resistor(s). Ion the lattercase, both types of heat sources contribute to the heating of theinfusion fluid. In this manner, the infusion fluid is warmed or heatedby excess heat generated by the portable energy source instead of beingwasted to the surrounding air. Consequently, efficient use is made ofthe energy stored in the portable energy source.

The housing shell may have a flat plate shaped structure with a heightless than 2.0 cm, preferably less than 1.0 cm.

Another aspect of the invention relates to an infusion fluid warmercomprising a casing shell having an upper wall structure and a lower,opposing, wall structure. A fluid channel or passage extends through thecasing shell in-between the upper and lower wall structures. Fluid inletand outlet ports are coupled to opposite ends of the fluid channel orpassage to allow a flow of infusion fluid through the casing shell. Aheating element is thermally coupled to the fluid channel to transferheat energy to the infusion fluid and the heating element comprises aportable energy source such as a rechargeable battery, anon-rechargeable battery, a super capacitor etc. This aspect of theinvention provides a fully portable infusion fluid warmer for a varietyof beneficial field uses for example in emergency or war zones allowinginfusion of fluids or blood without cooling the patient. The simplicityof the portable infusion warmer combined with small size and low weightare noticeable advantages for the medical personnel transporting thewarmer. The casing shell may comprise a thermoplastic material orelastomeric compound and fabricated by injection moulding. The casingshell may fully enclose or surround the heating element and the portableenergy source to protect these from mechanical shocks and impacts andpollutants in the external environment. Because the portable energysource is thermally coupled to the fluid channel, heat energy dissipatedtherein, for example due to internal impedance of the portable energysource, is conveyed to the infusion fluid to heat of the infusion fluidinstead of being wasted to the surrounding air. Consequently, energystored in the portable energy source is put to efficient use whichallows size, weight and energy storage capacity of the portable energysource to be reduced, or alternatively to provide higher energy storagecapacity of the portable energy source for a given size, weight orcapacity. The thermal coupling between the portable energy source energysource and the fluid channel is preferably provided by direct orindirect physical contact between these. In this context “direct orindirect physical contact” means in contact without any interveningpassage or layer of atmospheric air or other gaseous substances such asthrough an electrically insulating and thermally conducting energysource housing as described below. A thermal resistance between theportable energy source and the infusion fluid is preferably less than100° C./W, preferably less than 25° C./W, even more preferably less than10° C./W.

The portable energy source is preferably enclosed in a surroundingelectrically insulating and thermally conducting energy source housingwhich preferably is arranged inside the casing shell. In the latterembodiment, the energy source housing and the casing shell may becoaxially aligned to form a fluid channel surrounding the energy sourcehousing for example on all sides. In the latter embodiment, the fluidchannel extends around or encircles the entire perimeter of the energysource housing to maximize the transmission of heat energy from theportable energy source to the infusion fluid. At the same time,efficient liquid cooling of the portable energy source is achieved.

Each of the energy source housing and the casing shell may possess asubstantially circular, elliptical or rectangular cross-sectionalprofile. The fluid channel may accordingly be formed in-between an outersurface of the energy source housing and the upper and lower wallstructures of the casing shell. The skilled person will understand thatthe energy source housing may comprise anyone of the ceramic materialsdiscussed above in connection with the material of the housing shell.The heating element may additionally comprise a thin film resistor or athick film resistor bonded directly, for example by screen-printing, toa surface of the energy source housing facing away from the fluidchannel.

The energy source housing may be formed by a shell or casing of therechargeable or non-rechargeable battery or formed as a separate housingenclosing existing separate battery shell or shells to form a separatebattery compartment.

Each of the infusion fluid warmers described above in connection withthe first and second aspects of the invention may advantageouslycomprise a temperature sensor for determining a temperature of theinfusion fluid in the fluid channel at a suitable location such as at orproximate to the outlet port. A controller circuit may be operativelycoupled to the temperature sensor and to the heating element to controlinstantaneous power dissipation of the heating element. The controllercircuit is adapted to adjust power dissipation in the heating element inaccordance with a desired or target temperature of the infusion fluidbased on temperature data from the temperature sensor. The controllercircuit preferably comprises a programmable microprocessor such as aDigital Signal Processor and suitable program code or instructionsimplementing the control algorithm. The programmable microprocessor maybe an off-the-shelf industry standard type of microprocessor, preferablycomprising appropriate input and output ports and peripheral devicessuch as EEPROM or Flash memory. However, the skilled person willunderstand that the controller circuit alternatively may be implementedby appropriately configured programmable logic such as FPGA devices orhard-wired circuitry comprising combinatorial logic and memoryintegrated on an Application Specific Integrated Circuit (ASIC). Thecontroller circuit is preferably bonded to the surface of the upper wallstructure facing away from the fluid channel or the surface of the lowerwall structure facing away from the fluid channel. The controllercircuit may be arranged on the same surface of the upper or lower wallstructure as the thick film resistor or resistor if sufficient surfacearea is available. The use of the housing shell as carrier of thecontrol circuit in accordance with the above embodiments of theinvention provides a further size reduction of the infusion fluid warmerand reduces the number of separate parts that must be assembled duringmanufacturing of these infusion fluid warmers. In some embodiments ofthe invention, the controller circuit comprises one or moresemiconductor transistors and/or semiconductors diodes configured fordelivering a modulated drive signal to the heating element to dissipatepower therein. The semiconductor transistors may comprise one or morepower MOS transistors or IGBTs. The controller circuit controls thesemiconductor transistors to apply a PWM (Pulse Width Modulated) drivesignal across the heating element, e.g. the thick film resistor(s), suchthat a known quantity of electrical power is dissipated in the heatingelement. The controller circuit may be adapted to adjust the amount ofelectrical power dissipated in the heating element by adjusting the dutycycle of the PWM drive signal. Excess heat energy dissipated in the oneor more semiconductor transistors and/or semiconductors diodes of thecontroller circuit during operation of the heating element may also betransmitted to the fluid channel by bonding these semiconductors to thesurface of the upper and/or lower wall structure facing away from thefluid channel. The excess heat energy is caused by resistive andcapacitive parasitic losses in the one or more semiconductor transistorsand/or semiconductors diodes.

The temperature sensor(s) may comprise a semiconductor based sensor(s)placed in the fluid channel for example at or close to the outlet portsuch that a temperature of the heated or warmed infusion fluid can beaccurately measured. Infusion fluid temperature data may be transmittedto the controller circuit in digitally coded format or as an analoguevoltage, charge or current signal that is sampled by an analogue todigital converter (A/D-converter) in the controller circuit.

An advantageous embodiment of the temperature sensor comprises a thickfilm resistor or thin film resistor, preferably a thick or thin filmresistor of the heating element. Since the resistance of thick filmresistors is strongly temperature dependent these are particularlyuseful for temperature sensing and the controller circuit may be adaptedto measure the instantaneous resistance of the thick film resistor. Thecontroller can subsequently determine the temperature of the resistorfrom the determined instantaneous resistance by a suitable computingalgorithm or look-up table. Furthermore, if the thick film resistor andthe upper or lower wall structure of the housing shell has good thermalcontact with the fluid channel, the temperature of the thick filmresistor is approximately the same as the temperature of the infusionfluid such that the resistor temperature is a good estimate of theinfusion fluid temperature, possibly adjusted with a predeterminedcorrection factor.

Another, third aspect of the invention relates to a method of warminginfusion fluid during administration to a patient, comprising steps of:

-   -   providing an infusion fluid warmer according to any of the above        described aspects and embodiments thereof,    -   fastening the infusion fluid warmer to the patient for example        with a bracelet or tape,    -   connecting the outlet port to a Venflon or IV-catheter inserted        in the patient's vein,    -   connecting the inlet port to a fluid bag or container comprising        a volume of infusion fluid. If necessary, a short extension tube        may be inserted between the outlet port and the Venflon or        IV-catheter. Likewise, an extension tube may be coupled between        the inlet port and the fluid bag or container.

The ability to manufacture the present infusion fluid warmers with verycompact dimensions makes it possible to conveniently fasten the infusionfluid warmer directly on a patient's body for example on the leg or armwith a suitable adhesive device or substance such as tape, plaster,bandage, elastic band etc. This simplifies the fluid delivery processand minimizes risk of accidental detachment.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will be described in more detailin connection with the appended drawings, in which:

FIGS. 1a ) and 1 b) show a vertical cross-sectional view, and ahorizontal top view, respectively, of an infusion fluid warmer inaccordance with a first embodiment of the invention,

FIGS. 2a ) and 2 b) show a vertical cross-sectional view and ahorizontal top view, respectively, of an infusion fluid warmer inaccordance with a second embodiment of the invention,

FIGS. 2c ) and 2 d) show a vertical cross-sectional view and ahorizontal top view, respectively, of an infusion fluid warmer inaccordance with a third embodiment of the invention,

FIGS. 3a ) and 3 b) show a vertical cross-sectional view and ahorizontal top view, respectively, of an infusion fluid warmer inaccordance with a fourth embodiment of the invention,

FIGS. 4a ) and 4 b) show a vertical cross-sectional view and ahorizontal top view, respectively, of an infusion fluid warmer inaccordance with a fifth embodiment of the invention; and

FIG. 5 shows a vertical cross-sectional view of a battery poweredinfusion fluid warmer in accordance with a sixth embodiment of theinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1a ) shows a vertical cross-sectional view of an infusion fluidwarmer 100 in accordance with a first embodiment of the invention. Theinfusion fluid warmer 100 comprises a housing shell 104 formed in athermally conducting and electrically insulating material, preferablycomprising a ceramic material such as Aluminium Oxide (Al₂O₃). Thedimensions of the housing shell 104 may vary in accordance with specificrequirements for the infusion fluid warmer 100, in particular its fluidwarming capacity.

The housing shell 104 is encapsulated or enclosed within an outer casing102 which may be formed in a suitable polymeric material for example athermoplastic material or elastomeric compound by injection moulding.The outer casing 102 may be shaped and sized to protect the housingshell 104 from mechanical shocks and impacts. A pair of cap nuts or caps112, 114 covers respective entrance openings of the outer casing 102 andis preferably used to seal or isolate the interior volume of the outercasing 102 from liquids, dust and other pollutants in the externalenvironment. A prototype of the depicted infusion fluid warmer 100 wasproduced with a outer casing 102 with a 5.1 cm length, 3.3 cm width andthickness of 2.1 cm. The housing shell 104 has a plate-shaped formcomprises an upper wall structure 107 and a lower, opposing, wallstructure 105 divided by a fluid channel or passage 103 of meanderingshape projecting in a vertical plane, i.e. a plane perpendicularly tothe horizontal plane parallel with the outer surfaces of the housingshell 104. The fluid channel 103 extends through the housing shell 104in-between the upper and lower wall structures, 105, 107, respectively.In the present embodiment, the upper wall structure 107 and the lowerwall structure 105 are formed in separate upper and lower housing shellsbonded to each other by suitable means such as gluing, soldering,press-fitting, welding etc. A fluid channel 103 extends between a fluidinlet port 110 and a fluid output port 108 to allow a flow of infusionfluid through the housing shell 104. The fluid inlet port 110 is coupledto a first end of the fluid channel 103 and the fluid output portcoupled to an opposite end of the fluid channel 103. The fluid channelhas a meandering shape in the depicted vertical cross-sectional planeextending substantially perpendicularly to a flow of infusion fluid(indicated by arrows 111) at the inlet and outlet ports 110, 108,respectively. Cold or unheated infusion fluid such as blood or IVsolution flows from a fluid source such as fluid bag through an IV lineor tube 118 through the fluid inlet port 110, through the fluid channel103 at out of the fluid outlet port 108. From the fluid outlet port 108,heated or warmed infusion fluid flows through the IV line or tube 116towards an IV-catheter (e.g. Venflon) inserted in a patient's vessel forthe purpose of intravenous therapy.

The meandering shape of the fluid channel 103 is made by a pair ofmating grooves or trenches formed in the upper and lower wallstructures, 107, 105, respectively, of the housing shell 104. Heatenergy is therefore transferred to the infusion fluid in the fluidchannel 103 by direct contact with the heated wall structures of thehousing shell 104. The use of a bio-compatible ceramic material likeAluminium Oxide allows the infusion fluid to be in direct physicalcontract with the housing shell material in the fluid channel 103 andensures efficient heat transfer together with a simplified structure ofthe infusion fluid warmer with few separate parts. An array of thickfilm resistors 106 a-106 f acts like a heating element of the infusionfluid warmer 100 and are screen-printed on surfaces of the upper andlower wall structures, 105, 107, respectively facing away from the fluidchannel 103. Through application of a PWM (Pulse Width Modulated) drivesignal across the array thick film resistors, electrical power isdissipated therein so as to heat the thick film resistors as explainedin further detail below. Thick film resistors 106 a, 106 b and 106 c arescreen-printed on an outer surface of the upper wall structure 107facing oppositely to the fluid channel 103 and thick film resistors 106d, 106 e and 106 f are screen-printed on an outer surface of the lowerwall structure 107 facing oppositely to the fluid channel 103. Since thearray of thick film resistors 106 a-106 f are in good physical contactwith the upper and lower wall structures, 107, 105, respectively,without any intervening air gaps efficient thermal coupling is providedto infusion fluid flowing in the fluid channel 103 such that heat energyis transferred to the infusion fluid to warm the fluid. The thick filmresistors preferably cover a large portion of the respective outersurface areas which ensure a good thermal coupling between the resistorsand the upper and lower wall structures, 105, 107, respectively, of thehousing shell 104. The total resistance of the array of thick filmresistors 106 a-106 f as seen by the PWM (Pulse Width Modulated) drivesignal preferably lies between 0.001 ohm and 6250 ohm such as between0.1 ohm and 1 Kohm (10³ ohm).

The properties of the ceramic material used for the housing shell 104 inthe present embodiment, lead to several significant advantages. Oneadvantage is that the ceramic material has good thermal conductivitysuch that heat energy produced in the heating element (i.e. the thickfilm resistor array) is transferred to the infusion fluid with lowenergy loss. The ceramic material furthermore serves as a carrier of theheating element itself and finally serves as an electrical insulatorinsulating the PWM voltage applied to the heating element from theinfusion fluid and therefore from the patient.

The infusion fluid warmer 100 preferably comprises a temperature sensor(not shown) for determining a temperature of the infusion fluid in thefluid channel 103 for example at the outlet port 108 to ascertain theinfusion fluid temperature lies within a certain allowable range forexample between 36 and 37 degree Celsius. A controller circuit (notshown) is operatively coupled to the temperature sensor and to the arrayof thick film resistors 106 a-106 f to control instantaneous powerdissipation in the array. The instantaneous power dissipation in thearray of thick film resistors 106 a-106 f is preferably controlled byadjusting a duty cycle of the previously mentioned PWM drive signalapplied to the resistor array in accordance with a desired or targettemperature of the infusion fluid. The adjustment may for example beeffected through a suitable feedback loop and control algorithm executedby the controller circuit based on temperature data from the temperaturesensor. The controller circuit preferably comprises a programmablemicroprocessor such as a Digital Signal Processor and suitable programcode or instructions implementing the control algorithm. Theprogrammable microprocessor may be an off-the-shelf industry standardtype of microprocessor, preferably comprising appropriate input andoutput ports and peripheral devices such as EEPROM or Flash memory.However, the skilled person will understand that the controller circuitalternatively may be implemented by appropriately configuredprogrammable logic such as FPGA devices or hardwired circuitrycomprising a combination of combinatorial logic and memory integrated onan Application Specific Integrated Circuit (ASIC). The controllercircuit is preferably bonded to the outer surface of the upper wallstructure 107 or the outer surface of the lower wall structure 105 suchthat it is arranged adjacent to the thick film resistors 106 a, 106 band 106 c (please refer to FIG. 1b )) or the thick film resistors 106 d,106 e and 106 f. In this manner, the upper wall structure 107 or thelower wall structure 105 also functions a carrier for the controllercircuit and may be thermally coupled thereto such that heat energydissipated in the controller can be directed through the housing shell104 to the infusion fluid. The controller circuit is preferably poweredby rechargeable or non-rechargeable batteries such that the entireinfusion fluid warmer 100 is portable. In the alternative, thecontroller circuit may be energized by a mains operated power supplysuch as a switched mode power supply.

FIG. 1b ) shows a top view of the infusion fluid warmer 100 inaccordance with the first embodiment of the invention. The lid, or upperwall of the thermoplastic casing 102 has been removed such that thesurface of the upper wall 107 facing away from the fluid channel of thehousing shell 104 is visible. The thick film resistors 106 a, 106 b and106 c are disposed on the outer surface of the upper wall 107 covering alarge portion of the available surface area as previously mentioned.Each of the thick film resistors 106 a, 106 b and 106 c comprises aplurality of resistor segments arranged below each other. These resistorsegments may be coupled in series or parallel to provide a desiredresistance value of the thick film resistor. The skilled person willunderstand that the plurality of resistor segments could be replacedwith a single resistor of equivalent resistance. Likewise, the thickfilm resistors 106 a, 106 b and 106 c could be replaced with a singleresistor of equivalent resistance. As illustrated, a peripheral edgeportion is left uncovered by the thick film resistors 106 a, 106 b and106 c and may be used for attachment of the controller circuit providinga very compact and mechanical robust overall structure with individualcomponents placed in good thermal contact.

FIG. 2a ) shows a vertical cross-sectional view of an infusion fluidwarmer 200 in accordance with a second embodiment of the invention.Similar features in the present and the above-described first embodimentof the infusion fluid warmer have been provided with correspondingreference numerals to ease comparison. The infusion fluid warmer 200comprises a housing shell 204 formed in a thermally conducting andelectrically insulating material, preferably comprising a ceramicmaterial such as Aluminium Oxide (Al₂O₃). The dimensions of the housingshell 204 may vary in accordance with specific requirements for theinfusion fluid warmer 200, in particular its fluid warming capacity. Thehousing shell 204 is encapsulated or enclosed within an outer orexterior casing shell 202 held together by an associated pair of capnuts or caps 212, 214. The latter may be formed in one of the previouslydescribed materials and for the same purpose. The exterior casing shell202 comprises a fluid inlet port 210, where the infusion fluid enters(illustrated by arrow 211), and an oppositely arranged fluid outlet port208, where the infusion fluid exits (illustrated by arrow 213), duringoperation of the infusion fluid warmer 200

The housing shell 204 has a plate-shaped form which comprises the upperwall structure 207 and the lower, opposing, wall structure 205 separatedby a fluid channel or passage 203 extending between facing surfaces ofthe upper and lower wall structures 207, 205. In the present embodiment,the upper wall structure 207 and the lower wall structure 205 are formedin separate upper and lower housing shells which have been bonded toeach other after fabrication by suitable means such as gluing,soldering, press-fitting, welding etc. The fluid channel 203 has asubstantially straight horizontal shape in the present embodimentcompared to the meandering shape in the above-described first embodimentof the present infusion fluid warmer. The fluid channel 203 preferablyextends below a substantial portion of the facing surfaces of the upperand lower wall structures 207, 205 to maximize a flow rate of theinfusion fluid flow through the fluid channel 203. Likewise, the directphysical contact area between the infusion fluid and the upper and lowerwall structures 207, 205 through the facing surfaces is maximized toimprove direct heat energy transfer to the infusion fluid through theupper and lower wall structures 207, 205. The height of the fluidchannel 203 preferably lies between 0.1 mm and 5 cm. A significantadvantage of the straight channel or passage design chosen for thepresent embodiment is a smaller pressure drop. The fluid channel 203 maycomprise a plurality of vertical pillars mechanically connecting theupper wall structure 207 and the lower wall structure 205 to improve themechanical strength of the housing shell 204, in particular the strengthto vertically oriented shocks.

An array of thick film resistors 206 a-206 d acts as a heating elementof the infusion fluid warmer 200. The array of thick film resistors 206a-206 d is screen-printed, or firmly bonded or attached with analternative fasting mechanism, on surfaces of the upper and lower wallstructures, 207, 205, respectively, facing away from the fluid channel203. The array of thick film resistors is preferably bonded to the outersurfaces of the upper and lower wall structures 207, 205 such thatefficient thermal coupling, i.e. low thermal resistance, is achieved tothe wall structures and therefore also to the infusion fluid flowing inthe fluid channel 203. The individual resistors of the array of thickfilm resistors are preferably arranged in abutment with the wallstructure in question without any intervening air gap. In this manner,heat energy dissipated in the array of thick film resistors 206 a-206 dis efficiently transferred to the infusion fluid to warm the fluid. Thethick film resistors preferably cover a large portion of the respectivesurface areas which also ensure good thermal coupling between the thickfilm resistors and the upper and lower wall structures 207, 205respectively, of the housing shell 204. The total resistance of thearray of thick film resistors 206 a-206 d as seen by a drive signal suchas a PWM (Pulse Width Modulated) drive signal preferably lies between0.001 ohm and 6250 ohm such as between 0.1 ohm and 1 kΩ (10³ ohm).

The infusion fluid warmer 200 may comprise a temperature sensor (notshown) for determining a temperature of the infusion fluid in the fluidchannel 203 for example at the outlet port 208. The temperature sensormay be utilized to ascertain the infusion fluid temperature lies withina certain allowable range for example between 36 and 37 degree Celsius.A controller circuit (not shown) is operatively coupled to thetemperature sensor and to the array of thick film resistors 206 a-206 dto control instantaneous power dissipation in the array in the samemanner as described above in connection with the first infusion fluidwarmer embodiment 100.

The skilled person will appreciate the firm bonding between the array ofthick film resistors 206 a-206 d and the housing shell 204 provides acompact unitary assembly of heating element and heat exchanger with alow parts count in the present infusion fluid warmer 200.

FIG. 2b ) shows a top view of the infusion fluid warmer 200 inaccordance with the second embodiment of the invention. The lid or upperwall of the thermoplastic casing 202 has been removed such that theouter surface of the upper wall 207 of the housing shell 204 is exposed.The thick film resistors 206 a, 206 b are disposed on the outer surfaceof the upper wall 207 covering a large portion of the available surfacearea. Each of the thick film resistors 206 a, 206 b comprises aplurality of individual resistor segments arranged in width wisedirection. These resistor segments may be coupled in series or parallelto provide a desired resistance value of the thick film resistor inquestion.

FIG. 2c ) illustrates a vertical cross-sectional view of an infusionfluid warmer 250 in accordance with a third embodiment of the invention.Similar features and elements of the present embodiment and theabove-described second embodiment of the infusion fluid warmer have beenprovided with corresponding reference numerals to ease comparison. Anexterior casing shell 252 comprises a fluid inlet port 260, where theinfusion fluid enters (illustrated by arrow 261), and an oppositelyarranged fluid outlet port 258, where the infusion fluid exits enters(illustrated by arrow 263), during operation of the infusion fluidwarmer 250. The housing shell 254 is encapsulated or enclosed within theouter or exterior casing shell 552 held together by an associated pairof cap nuts or caps 262, 264.

In the present infusion fluid warmer 250, the fluid channel comprises afirst or upper channel segment 253 a arranged between the upper wallstructure 257 of the housing shell 254 and the upper wall structure ofthe exterior casing 252. A second channel segment 253 b is arrangedbetween the lower wall structure 255 of the housing shell 254 and aninwardly oriented surface of the lower wall structure of the exteriorcasing 252. In this manner, the fluid channel extends around the housingshell 254 instead of through the housing shell 204 as in the secondembodiment described above. The upper and the lower wall structures 257,255, may be separate parts that have been bonded after theirfabrication. The housing shell 204 is preferably formed in a thermallyconducting and electrically insulating material, preferably comprising aceramic material such as Aluminium Oxide (Al₂O₃). The upper and lowerfluid channels 253 a, 253 b preferably extends below a substantialportion of the facing surfaces of the upper and lower exterior casing tomaximize a flow rate of the infusion fluid flow through the fluidchannels. Likewise, the direct physical contact between the infusionfluid and the upper and lower wall structures 257, 255 through thefacing surfaces is maximized to improve direct heat energy transfer tothe infusion fluid through the upper and lower wall structures 257, 255.The straight channel or passage design of the upper and lower fluidchannels 253 a, 253 b leads to a small pressure drop.

An array of thick film resistors 256 a-256 d acts as a heating elementof the infusion fluid warmer 250. The array of thick film resistors 206a-206 d is screen-printed, or firmly bonded or attached with analternative fasting mechanism, on surfaces of the upper and lower wallstructures, 257, 255, respectively, facing away from the fluid channels253 a, 253 b. The upper array of thick film resistors 206 a-206 b isbonded to the upper wall structure 257 and faces the lower array ofthick film resistors 206 c-206 d bonded to the lower wall structure 255such that a small intermediate volume is formed there between. The upperand lower wall structures, 257, 255 u are preferably sealing bonded toeach other such that infusion fluid is prevented from entering the smallintermediate volume and short-circuit the thick film resistors. Thearray of thick film resistors 256 a-256 d is preferably bonded to theupper and lower wall structures 207, 205 such that efficient thermalcoupling, i.e. low thermal resistance, is achieved to the respectivewall structures and therefore also to the infusion fluid flowing in theupper and lower fluid channels 253 a, 253 b. As described above, theindividual resistors of the array of thick film resistors are preferablyarranged in abutment with the wall structure in question without anyintervening air gap to ensure efficient heat transfer from the array ofthick film resistors 206 a-206 d through the upper and lower wallstructures, 257, 255 to the infusion fluid.

The skilled person will appreciate the firm bonding between the array ofthick film resistors 256 a-256 d and the housing shell 254 provides acompact unitary assembly of heating element and heat exchanger with alow parts count in the present infusion fluid warmer 250.

FIG. 2d ) shows a top view of the infusion fluid warmer 250 inaccordance with the third embodiment of the invention. The lid or upperwall of the thermoplastic casing 252 has been removed such that theouter surface of the upper wall 257 of the housing shell 254 is exposed.The thick film resistors 206 a, 206 b are disposed on the inner surfaceof the upper wall 207 facing away from the upper fluid channel. Thethick film resistors 206 a, 206 b are covering a large portion of theavailable surface area of the upper wall 257. Each of the thick filmresistors 206 a, 206 b comprises a plurality of individual resistorsegments arranged in width wise direction. These resistor segments maybe coupled in series or parallel to provide a desired resistance valueof the thick film resistor in question.

FIG. 3a ) shows a vertical cross-sectional view of an infusion fluidwarmer 300 in accordance with a third embodiment of the invention.Similar features in the present embodiment and the above-described firstembodiment of the infusion fluid warmer have been provided withcorresponding reference numerals to ease comparison. The infusion fluidwarmer 300 comprises a housing shell 304 formed in a thermallyconducting and electrically insulating material, preferably comprising aceramic material such as Aluminium Oxide (Al₂O₃). The dimensions of thehousing shell 304 may vary in accordance with specific requirements forthe infusion fluid warmer 300, in particular its fluid warming capacity.The housing shell 304 is encapsulated or enclosed within an outer casing302 and its associated pair of cap nuts or caps 312, 314 which may beformed in one of the previously described materials and for the samepurpose. The housing shell 304 comprises a plate shaped upper wallstructure 307 and a separate lower, opposing, plate shaped wallstructure 305. An aluminium heat exchanger 309 is sandwiched in-betweenthe plate shaped upper and lower wall structures 307, 305, respectively,and thermally coupled thereto for example by direct physical contact orthrough a layer of a suitable thermal compound. A fluid channel 303 isformed in the metallic heat exchanger 309 such that infusion fluid isbrought in physical contact with the metallic heat exchanger to receiveheat energy therefrom. The upper wall structure 307 and the lower wallstructure 305 may be bonded or fastened to opposing sides of themetallic heat exchanger 309 by any suitable means and manufacturingprocesses such as gluing, soldering, press-fitting, welding etc. Thickfilm resistors 306 a, 306 b and 306 c are screen-printed on an outersurface of the upper wall structure 307 facing oppositely to themetallic heat exchanger 309 and thick film resistors 306 d, 306 e and306 f are screen-printed on an outer surface of the lower wall structure307 facing oppositely to the metallic heat exchanger 309 in which afluid channel 303 is formed. The fluid channel 303 extends between afluid inlet port 310 and a fluid outlet port 308 arranged in themetallic heat exchanger 309. The fluid channel 303 has a meanderingshape similar to the shape in the above-described first embodiment ofthe infusion fluid warmer. The fluid channel 303 preferably extendsbelow a substantial portion of the width of the upper and lower wallstructures 307, 305 to improve an infusion fluid flow rate through thefluid channel 303. Likewise, the direct physical contact area betweenthe infusion fluid and the upper and lower wall structures 307, 305inside the fluid channel 303 is maximized to improve heat energytransfer. The height of the fluid channel 303 preferably lies between0.1 mm and 5 cm. A significant advantage of the metallic heat exchanger309 is its lower thermal resistance compared to e.g. ceramic materialwhereby the fluid warming capacity is increased allowing higher fluidflow rate. The metallic heat exchanger 309 also increases the surfacearea and the mechanical strength of the housing shell 304 albeit at theexpense of size and complexity of the infusion fluid warmer.

FIG. 3b ) shows a top view of the infusion fluid warmer 300 inaccordance with the third embodiment of the invention. The lid, or upperwall of the thermoplastic casing 302 has been removed such that theouter surface of the upper wall 307 of the housing shell 304 is exposed.Edge segments of the metallic heat exchanger 309 protrude or projectlengthwise to the outside of the upper surface 307 of the ceramichousing shell 304. The fluid inlet port 310 is formed beneath the leftside edge segment 309 a and the fluid outlet port is formed beneath theright side edge segment 309 b. The thick film resistors 306 a, 306 b,306 c are disposed on the outer surface of the upper wall 307 covering alarge portion of the available surface area. Each of the thick filmresistors 306 a, 306 b, 306 c comprises a plurality of individualresistor segments arranged in width wise direction. These resistorsegments may be coupled in series or parallel to provide a desiredresistance value of the thick film resistor in question. Since the thickfilm resistors 306 a, 306 b, 306 c are in good thermal contact with theupper wall structure 307, which in turn is in good thermal contact withthe metallic heat exchanger 309, heat energy dissipated in the thickfilm resistors 306 a, 306 b, 306 c is efficiently transmitted or coupledto the infusion fluid in the fluid channel 303.

FIG. 4a ) shows a vertical cross-sectional view of an infusion fluidwarmer 400 in accordance with a forth embodiment of the invention.Similar features in the present embodiment and the above-described firstembodiment of the infusion fluid warmer have been provided withcorresponding reference numerals to ease comparison. The infusion fluidwarmer 400 comprises a housing shell 404 formed in a thermallyconducting and electrically insulating material, preferably comprising aceramic material such as Aluminium Oxide (Al₂O₃). The dimensions of thehousing shell 404 may vary in accordance with specific requirements forthe infusion fluid warmer 400, in particular its fluid warming capacity.The housing shell 404 is encapsulated or enclosed within an outer casing402 and its associated pair of cap nuts or caps 412, 414 which may beformed in one of the previously described materials and for the samepurpose. The housing shell 404 comprises a rectangular plate shapedupper wall structure 407 and a separate rectangular lower, opposing,plate shaped wall structure 405. An aluminium heat exchanger 409 issandwiched in-between the plate shaped upper and lower wall structures407, 405, respectively, and thermally coupled thereto for example bydirect physical contact or through a layer of suitable thermal compound.The aluminium heat exchanger 409 has a rectangular structure with flatupper and lower surfaces mating to the plate shaped upper and lower wallstructures 407, 405 to allow good thermal coupling (i.e. low thermalresistance) between these parts. A straight fluid channel 403 withrectangular cross-sectional profile extends centrally through themetallic heat exchanger 409 such that infusion fluid is brought inphysical contact with the metallic heat exchanger 409 to receive heatenergy therefrom. The upper wall structure 407 and the lower wallstructure 405 may be bonded or fastened to opposing sides of themetallic heat exchanger 409 by any suitable fastening process such asgluing, soldering, press-fitting, welding etc. Thick film resistors 406a, 406 b are screen-printed on an outer surface of the upper wallstructure 407 facing oppositely to the metallic heat exchanger 409 andthick film resistors 406 c, 406 d are screen-printed on an outer surfaceof the lower wall structure 405 facing oppositely to the metallic heatexchanger 409 in which a fluid channel 403 is formed. The fluid channel403 extends between a fluid inlet port or slit 410 and a fluid outletport or slit 408 arranged in the metallic heat exchanger 409. The fluidchannel 403 preferably extends below a substantial portion of the widthof the upper and lower wall structures 407, 405 to improve an infusionfluid flow rate through the fluid channel 403. Likewise, a directphysical contact area between the infusion fluid and the upper and lowerwall structures of the fluid channel 403 is maximized to improve heatenergy transfer. The height of the fluid channel 403 preferably liesbetween 0.1 mm and 5 cm. A significant advantage of the straight channelform of the present metallic heat exchanger 409 compared to theabove-described meandering fluid channel shape in the metallic heatexchanger 309 is its smaller pressure drop. The metallic heat exchanger409 also increases the surface area and the mechanical strength of thehousing shell 404.

FIG. 4b ) shows a top view of the infusion fluid warmer 400 inaccordance with the fourth embodiment of the invention. The lid or upperwall structure of the thermoplastic casing 402 has been removed suchthat the outer surface of the upper wall 407 of the ceramic housingshell 404 is exposed. The upper wall 407 covers the underlying metallicheat exchanger 403. The fluid inlet port 410 is formed as a rectangularslit in a central portion of the metallic heat exchanger 409 and thefluid outlet port 408 is likewise formed as a rectangular slit in thecentral portion of the metallic heat exchanger 409 in its opposite end.The thick film resistors 406 a, 406 b are disposed, preferablyscreen-printed, on the outer surface of the upper wall 407 covering alarge portion of the available surface area. Each of the thick filmresistors 406 a, 406 b comprises a plurality of individual resistorsegments arranged in width wise direction. These resistor segments maybe coupled in series or parallel to provide a desired resistance valueof the thick film resistor in question. Since the thick film resistors406 a, 406 b are in good thermal contact with the upper wall structure407, which in turn is in good thermal contact with the metallic heatexchanger 409, heat energy dissipated in the thick film resistors 406 a,406 b is efficiently transmitted to the infusion fluid in the fluidchannel 403. Naturally, the same kind of efficient heat energy transferis made from the thick film resistors 406 c, 406 d to the infusionfluid.

FIG. 5 shows a vertical cross-sectional view of a battery poweredinfusion fluid warmer 500 in accordance with the previously discussedsecond and separate aspect of the invention wherein the heating elementcomprises a portable energy source such as one or more rechargeablebatteries, non-rechargeable batteries, super capacitors etc. Theinfusion fluid warmer 500 comprises an outer casing 502 which may beformed in a suitable polymeric material for example a thermoplasticmaterial or elastomeric compound by injection moulding. The outer casing502 may be shaped and sized to protect a battery housing or shell 504from mechanical shocks and impacts. A pair of cap nuts or caps 512, 514covers respective entrance openings of the outer casing 502 and ispreferably used to seal or isolate the interior volume of the outercasing 502 from liquids, dust and other pollutants in the externalenvironment. The dimensions of the outer casing 502 may vary inaccordance with specific requirements for the infusion fluid warmer 500,in particular its fluid warming capacity.

The outer casing 502 has a cylindrical shape with a semi-cylindricalupper wall structure 507 and a lower, opposing, semi-cylindrical wallstructure 505. The battery shell 504 is preferably formed in a thermallyconducting and electrically insulating material such as a ceramicmaterial for example Aluminium Oxide (Al₂O₃) to electrically insulatethree rechargeable batteries 519 a, 519 b, 519 c from a fluid channel503. The battery shell 504 preferably has a cylindrical shape conformingto an inner contour of the outer casing 502 but with a cross-sectionaldiameter sufficiently small to leave an annular cylindrical passage 503between the inner surface of the outer casing 502 and the battery shell504. The annular cylindrical passage 503 forms a fluid channel orpassage extending horizontally between a fluid inlet port 510 and afluid output port 508 to allow a flow of infusion fluid through theouter casing 502. The skilled person will understand that the threerechargeable batteries 519 a, 519 b, 519 c could be have numerous othershapes than cylindrical and the shapes of the battery shell 504 andouter casing 502 adapted thereto. In one such embodiment, each of therechargeable batteries has a rectangular cross-sectional profile and thefluid channel 503 has a corresponding cross-sectional profile. Theskilled person will understand that the separate battery shell 504 whichencloses the three rechargeable batteries may be superfluous in otherembodiments of the portable infusion fluid warmer. In these otherembodiments, each outer casing of the rechargeable batteries may possessa thermally conducting and electrically insulating property andtherefore brought in direct contact with the infusion fluid. The cold orunheated infusion fluid such as blood or IV solution flows from a fluidsource such as fluid bag through an IV line or tube 518 through thefluid inlet port 510, through the fluid channel 503 at out of the fluidoutlet port 508. From the fluid outlet port 508, heated or warmedinfusion fluid flows through the IV line or tube 516 towards anIV-catheter (e.g. Venflon) inserted in a patient's vessel for thepurpose of intravenous therapy. The rechargeable batteries 519 a, 519 b,519 c act like heating elements in accordance with the presentembodiment of the invention. The outer surface of the battery shell 504conducts heat energy generated by the batteries to the infusion fluid inthe fluid channel 503 because of the direct physical contact between theouter surface of the battery shell 504 and the (flowing) infusion fluid.The rechargeable batteries 519 a, 519 b, 519 c are preferably in goodthermal contact with the battery shell 504 for example by means of adirect physical contact or by means of a thermal contact through anintervening layer of thermal compound or an intervening layer of solidmaterial with good thermal conductivity.

The infusion fluid warmer 500 preferably comprises a temperature sensor(not shown) for determining a temperature of the infusion fluid in thefluid channel 503 for example at the outlet port 508 to ascertain theinfusion fluid temperature lies within a certain allowable range forexample between 36 and 37 degree Celsius. A controller circuit (notshown) is operatively coupled to determine or set a discharge currentdrawn from the 3 rechargeable batteries 519 a, 519 b, 519 c so as tocontrol the instantaneous amount of power dissipated internally withinthe rechargeable batteries 519 a, 519 b, 519 c due to their respectiveinternal impedances. This internal power dissipation in the rechargeablebatteries 519 a, 519 b, 519 c leads to the generation of heat energywhich is thermally coupled to the infusion fluid through the thermallyconducting battery shell 504 as explained above. In this manner, theinfusion fluid is warmed or heated by excess heat generated by therechargeable batteries 519 a, 519 b, 519 c instead of wasting excessheat to the surrounding air such that efficient use is made of energystored in the rechargeable batteries 519 a, 519 b, 519 c. The controllercircuit may use temperature data from the temperature sensor to controlthe instantaneous power dissipation internally within the rechargeablebatteries such that a desired infusion fluid temperature at the outletport is maintained during delivery of the infusion fluid to the patient.

The skilled person will understand that the above-outlined use ofinternal power dissipation of the rechargeable batteries 519 a, 519 b,519 c to heat the infusion fluid may be supplemented with a separateheating element such as the previously described arrays of thick filmresistors. The array of thick film resistors could be screen-printed ona suitable inner surface area (i.e. on the opposite side of the fluidchannel) of the battery shell 504 such as to be electrically insulatedfrom the infusion fluid, in particular if the battery shell materialcomprises a suitable ceramic material.

In this manner, efficient use is made of both power dissipated in thearray of thick film resistors and internal power dissipation in therechargeable batteries. The controller circuit preferably comprises aprogrammable microprocessor such as a Digital Signal Processor andsuitable program code or instructions implementing the control algorithmas previously described.

The invention claimed is:
 1. An infusion fluid warmer comprising: acasing shell having an upper wall structure and a lower, opposing, wallstructure; said casing shell enclosing: a fluid channel extendingthrough the casing; fluid inlet and outlet ports coupled to oppositeends of the fluid channel to allow a flow of infusion fluid through thecasing shell; a housing shell formed in a thermally conducting andelectrically insulating material and comprising a plate shaped upperwall structure and an opposing plate shaped lower wall structure; afirst heating element bonded to the plate shaped upper wall structureand thermally coupled thereto; a second heating element bonded to theplate shaped lower wall structure and thermally coupled thereto, and analuminum heat exchanger sandwiched between the plate shaped upper wallstructure and the opposing plate shaped lower wall structure andthermally coupled to the plate shaped upper wall structure and plateshaped lower wall structure, wherein a straight portion of the fluidchannel possesses a rectangular cross-sectional profile and extendsthrough the aluminum heat exchanger such that heat energy is transferredto the infusion fluid by direct physical contact with aluminum heatexchanger material, wherein the first heating element is bonded to asurface of the plate shaped upper wall structure facing away from thestraight portion of the fluid channel and the second heating element isbonded to a surface of the plate shaped lower wall structure facing awayfrom the fluid channel, and wherein at least one surface of the plateshaped upper and lower wall structures facing away from the fluidchannel comprises a pair of electrical coupling terminals for receipt ofelectrical power to the first heating element or the second heatingelement.
 2. An infusion fluid warmer according to claim 1, wherein thefirst and second heating elements comprise portable energy sources. 3.An infusion fluid warmer according to claim 2, wherein a thermalresistance between the portable energy source and the fluid channel isless than 100° C./W.
 4. An infusion fluid warmer according to claim 2,wherein a thermal resistance between the portable energy source and thefluid channel is less than 25° C./W.
 5. An infusion fluid warmeraccording to claim 2, wherein a thermal resistance between the portableenergy source and the fluid channel is less than 10° C./W.
 6. Aninfusion fluid warmer according to claim 1, wherein the housing shellhas a flat plate shaped structure with a height less than 2.0 cm.
 7. Aninfusion fluid warmer according to claim 1, wherein the housing shellcomprises a ceramic material selected from the group consisting ofAluminum oxide (Al₂O₃), Aluminum Nitrate, and Beryllium Oxide.
 8. Aninfusion fluid warmer according to claim 1, wherein the first heatingelement or the second heating element comprises a thick film or thinfilm resistor.
 9. An infusion fluid warmer according claim 8, whereinthe temperature sensor comprises a thick film resistor or thin filmresistor of the first or second heating elements.
 10. An infusion fluidwarmer according to claim 1, wherein the housing shell has a flat plateshaped structure with a height less than 1.0 cm.
 11. An infusion fluidwarmer according to claim 1, wherein a height of the straight portion ofthe fluid channel is between 0.1 mm and 5 cm.
 12. An infusion fluidwarmer according to claim 1, further comprising: a temperature sensorfor determining a temperature of the infusion fluid in the fluidchannel; and a controller circuit operatively coupled to the temperaturesensor and to the first and second heating elements to controlinstantaneous power dissipation of the first and second heatingelements; wherein the controller circuit is adapted to adjust powerdissipation in the first and second heating elements in accordance witha desired or target temperature of the infusion fluid based ontemperature data from the temperature sensor.
 13. An infusion fluidwarmer according to claim 12, wherein the controller circuit comprisesone or more semiconductor transistors and/or semiconductors diodesdelivering a modulated drive signal to the first and second heatingelements to adjust the instantaneous power dissipated therein.
 14. Aninfusion fluid warmer according claim 12, wherein the temperature sensorcomprises a thick film resistor or thin film resistor of the first orsecond heating elements.